1. Field of the Invention
The present invention relates to LED modules in which LED (Light Emitting Diode) elements are mounted on a wiring board.
2. Description of Related Art
LED modules combine LED elements or LEDs (packaged LED elements for mounting) with wires for powering them, a wiring board, etc. A simple example of the LED module is one for illumination use that combines LEDs interconnected by general-purpose wires with a controller for turning on and off the LEDs.
In recent years, the luminous efficiency of LEDs has been improved and blue LEDs have been developed; therefore it has become possible to produce white light by using a blue LED with a phosphor, or by combining red, green and blue LEDs. Hence, these white light sources are widely used as a backlight of a small-size FPD (Flat Panel Display) for use in a cellular phone and the like. In addition, such LED technologies have been further improved to the point where LEDs can also replace fluorescent lamps conventionally used for a backlight of liquid crystal displays or a lighting device, and such applications have been put to commercial use.
The following are important points to consider when using LEDs for such applications: (1) light emitted in an LED element is efficiently utilized; (2) heat generated from an LED element is efficiently dissipated to restrain the LED element from elevating temperature; (3) an LED element can be performance tested; etc.
As a solution to the above point (1), a flip chip mountable LED element is proposed (e.g., JP-A Hei 11(1999)-340514). As a solution to the above point (2), there is a method in which an electrical insulator with a good thermal conductivity is used as a submount to increase heat conduction area (e.g., JP-A-2006-86139). A solution to the above point (3) can be provided by commercially available LEDs which are packaged to be mountable on a wiring board or the above-mentioned submount. Further, as a solution to both the above points (1) and (2), an LED element is proposed which is flip chip mountable and has electrically insulating bumps for heat dissipation (e.g., JP-A-2003-110148).
Most of the conventional mountings of LEDs use a submount in which a power supply metal wiring pattern is formed by a vapor growth method or the like on the surface of an electrically insulating ceramic with a good thermal conductivity such as aluminum nitride. However, the submount is a separate component from the wiring board for powering it, resulting in an increase in the number of constituent components of an LED module. In addition, such the electrically insulating ceramic materials are expensive and the manufacturing cost of forming the wiring on the submount surface using the vapor growth method is also high. And, all these factors have been an obstacle to cost reduction of LED modules.
Meanwhile, without using such the ceramic materials, it is difficult to provide both heat conduction and electrical insulation by, for example, bonding a heat dissipation plate to an LED module in which a wiring board and LED elements are directly flip chip connected to each other, since most of the electrically insulating materials used in wiring boards have a low thermal conductivity. For example, the thermal conductivity of electrically insulating polyimide is only 0.25 W/m·K compared with 398 W/m·K of electrically conductive copper.
Furthermore, there is another problem in that, when such a flip chip mounted module is subjected to a temperature cycle, a stress is induced, in and at its contacted surface of a connecting member such as a solder ball, by thermal expansion difference between an LED element and wiring board, which may destroy the LED element, or damage the connection member or its contacted surface.